1. Field of the Invention
This invention relates to a solid electrolyte storage battery or an oxygen pump. More specifically, it relates to a sodium-sulfur storage battery made of .beta.-Al.sub.2 O.sub.3 having a large storage capacity per unit weight, a cationically conductive crystalline solid separator used as a diaphragm between a cathode cell and an anode cell in an apparatus for electrolyzing molten sodium chloride to produce chlorine and sodium hydroxide, and to an oxygen ion-conductive crystalline solid separator based on stabilized zirconia.
2. Description of the Prior Art
In the applications described above, reaction is induced with good efficiency by using a number of thin, bottomed hollow cylindrical structures or plates of .beta.-alumina or stabilized zirconia arranged in parallel as a solid separator. In order to reduce the electric resistance of such a separator, the hollow cylindrical structures or plates must be reduced in thickness and increased in number. However, there is a practical limit to the reduction of the electric resistance of such a separator and increasing the surface area thereof by reducing the hollow cylindrical structures or plates in thickness and increasing them in number.
.beta.-alumina and stabilized zirconia as are used in the above-described applications are brittle, not only when they are green but also when been converted to sintered bodies. The production of bottomed hollow cylindrical structures or plates having a small thickness from these materials, therefore, is subject to the disadvantages that yield decreases in each step of molding, drying, firing, transportation and assembling, and handling structures thereof requires great care in order to prevent breakage.
For example, in the case of a solid electrolyte battery having a separator composed of an oxygen ion-conductive solid sintered electrolyte, and an oxygen pump made by applying to such a battery a voltage exceeding its open circuit voltage, the solid electrolyte separator usually operates at a high temperature, and generally a high temperature fuel cell (concentration cell) or a direct power generating device is obtained by permeation of oxygen ions through the separator based on partial pressure difference. Phrased differently, a positive potential is generated in areas under high oxygen partial pressure while a negative potential is generated in areas under low oxygen partial pressure. These potentials can be taken out by means of electrodes. Known materials for such separators are of the stabilized zirconia-type, thoria-type, ceria-type, bismuth oxide-type, ceria-lanthania-type, and ceria-thoria-lanthania-type.
The greatest difficulty with such solid electrolyte separators is that their electric conductivity is considerably lower than that of the corresponding molten salt. For this reason, when it is desired to obtain a current of 200 mA/cm.sup.2 by adjusting the thickness of the electrolyte separator to 1 mm, a separator resistance loss of, for example, 0.2 to 0.8 V is generated. Since the open circuit voltage of one fuel cell is about 1 V, this loss affects the feasibility of the cell. It has been desired, therefore, to develop a solid electrolyte having a higher electric conductivity, and to thin the solid electrolyte itself.
Known solid electrolytes are generally solid solutions. Accordingly, in view of their manufacturing difficulty and handling safety, the thickness of such solid electrolytes in conventional shapes such as a plate, hollow cylinder or tubing shape, used for safety purposes, is generally about 1 to 1.3 mm. A small circular plate having a thickness of 0.5 has been suggested, but building a commercial-scale cell from such small circular plates is complicated, and their handling requires meticulous care. An electrolyte separator having a thickness of as small as 0.1 mm has also been reported, but this can be produced only by flame spraying onto a metal. No single electrolyte structure having such a small thickness has been reported so far.
Even if this electrolyte structures can be obtained from prior art materials, complicated operations and processes must be performed in order to form them to a size for practical applications. With conventional techniques, therefore, it is not easy to obtain both a practical current density and a large total current (i.e., large separator area).
A storage battery or secondary cell having an alkali ion conductive thermally stable separator usually includes a anode reactant consisting essentially of an alkali metal and an cathode reactant consisting essentially of a compound of the alkali metal or an electrolyte containing an alkali metal compound. The separator is preferably a solid electrolyte separator. Alkali ion conductive glass is also known as a separator material.
As is known, the solid electrolytes mentioned above permits the selective passage of, for example, Na ions, when the anode and cathode reactants are in the molten state. By the selective passage of Na ions, the potential difference present between metallic sodium (anode cell) and an electrolyte (cathode cell) capable of reversibly reacting electrochemically with Na, for example, a sulfur-containing electrolyte (sodium sulfide) or a mixture of AlCl.sub.3 and SbCl.sub.3, which are disposed with a separator therebetween is maintained, thus forming a cell from an electrical output is generated.
A solid electrolyte as separator operates only at the high temperatures at which the metallic sodium and the electrolyte mentioned above are in the molten state. For the purpose of maintenance, cells which operate at lower temperatures have been desired. One example thereof is represented by the cell reaction which takes place in accordance with formula (A) below in the cathode cell, and formula (B) in the anode cell: ##STR1##
The cathode electrolyte (mixed salt) used in the above cell reaction has a low melting point, and can function at a lower temperature (200.degree. C.) than the temperature (at least 300.degree. C.) at which a conventional Na/S cell can operate. This is favorable for the corrosion resistance of the material forming the cell. Despite this advantage, this type of cell has the inherent defect that the solid electrolyte has a high electric resistance at lower temperatures, and, consequently, the voltage consumed inside the cell increases and the current generated for use decreases.
A limit on the dimensions of conventional alkali ion conductive solid electrolytes, such as .beta.-alumina, is generally imposed from the standpoint of production and handling thereof, and requires the use of a bottomed hollow cylindrical structure having a thickness of about 0.9 to 1.3 mm and a diameter of 10 to 15 mm. A small circular plate having a thickness of 0.4 mm has been reported, but a large-sized cell cannot be produced from such small circular plates. Moreover, a number of process steps are required to build a cell using such small circular plates as separators. To obtain larger areas, it is necessary to combine still more unit cells and such is difficult in practice.
It has been desired, therefore, to reduce the electrical resistance of solid electrolyte separators by thinning the same.
Furthermore, to obtain a high voltage and high current (high output), it has been greatly desired to simplify and reduce the entire apparatus, increase safety, and to reduce production costs.